Downward Message Processing implementation

[pepyakin]

Split off from #1679
Closes #1678
Related to #1806

Note that this change:

• includes updates of XCM, but doesn't include the changes The Router Module #1679. The current plan is to land the guide changes together.
• the last commit from The Router Module #1679 was split from the HRMP part and properly implemented.
• I didn't address the suggestion of @coriolinus since I don't feel the discussion is over. I think it is better to continue it here if my arguments are not conviencing or if there are better suggestions.

[rphmeier]

I'll leave the bikeshed up to you and @coriolinus

[coriolinus]

if I am coding some stuff that works with the downward messages, what should I take: downward message or a downward message contents?

I guess it depends what you want to do with it. To me, if you have DownwardMessage and DownwardMessageContents, then the implication is that the former wraps and possibly extends the latter.

That said, I don't want to hold anything up with bikeshedding; if you feel that the current naming scheme is better, then just go ahead with it as it is.

[pepyakin]

ok, then I am going to go ahead and merge. Although I'll try to keep this issue in mind when writing the client code.

[pepyakin]

bot merge

[ghost]

Trying merge.